Fuel cells that generate electricity through the electrochemical reaction between a fuel gas (e.g., hydrogen) and an oxidant gas (e.g., oxygen) are drawing attention as an energy source. The fuel cell is provided with an electricity generation body (e.g., a membrane-electrode assembly) formed by joining an anode and a cathode to two opposite sides of an electrolyte membrane (e.g., a solid polymer membrane that has proton conductivity). The anode of the electricity generation body is supplied with the fuel gas from a fuel gas channel, and the cathode is supplied with the oxidant gas from an oxidant gas channel.
In the case where a membrane-electrode assembly formed by joining an anode and a cathode to two opposite surfaces of an electrolyte membrane that has proton conductivity is used as the aforementioned electricity generation body, the oxidant gas supplied to the cathode of the membrane-electrode assembly is generally an air-containing oxygen. In the case where air is used as the oxidant gas, an impurity gas that is contained in air and does not contribute to the electricity generation, such as nitrogen or the like, permeates through the electrolyte membrane from the cathode side to the anode side.
Since the permeation of the impurity gas from the cathode side to the anode side continues to occur even during a stop of electricity generation, the impurity gas fills the anode in the case the electricity generation is stopped for a long time. Then, in so-called anode dead-end type fuel cells in which substantially the entire amount of the fuel gas supplied to the anode is used for electricity generation while residing within the fuel cells without being discharged to the outside of the fuel cells, the discharge of the anode off-gas to the outside of the fuel cells is not performed. Therefore, in the anode dead-end type fuel cell, if the fuel gas is supplied to the anode at the time of start-up of the fuel cell, the impurity gas in the anode comes to locally reside in a downstream region in the flowing direction of the fuel gas due to the flow of the fuel gas. Then, in the case where the fuel cell in this state is connected to a load and is caused to generate electricity, the concentration of the fuel gas becomes lower in a region in the membrane-electrode assembly in which the impurity gas is residing than in a region therein in which the impurity gas is not residing. As a result, the distribution of electricity generation in the membrane-electrode assembly becomes non-uniform, bringing about a decline in the electricity generation efficiency.
Let it considered that a fuel cell system that includes fuel cells described above is mounted as a power source of an electric vehicle. In this case, if electricity is generated with the impurity gas residing in a portion of the anode and electric power is supplied to the load, the cell voltage can decline, affecting the operation of the vehicle (system).
In a fuel cell stack in which a plurality of membrane-electrode assemblies are stacked with separators disposed therebetween, if the concentration of the fuel gas declines in any one of the membrane-electrode assemblies, the electromotive force of that membrane-electrode assembly declines, so that a reverse voltage occurs between the anode and the cathode of the membrane-electrode assembly. Generally, the anodes and the cathodes of the membrane-electrode assemblies are each constructed of a catalyst layer and a gas diffusion layer. In many cases, a carbon supporting a catalyst metal is used in the catalyst layer. Therefore, if a reverse voltage occurs between the anode and the cathode of a membrane-electrode assembly, there arises a problem of the anode-side catalyst layer of that membrane-electrode assembly degrading due to oxidation of the carbon. This carbon oxidation is represented by equation (1).C+2H2O→CO2+4e−+4H+  (1)